Dynamic selection of a compression algorithm for trace data

ABSTRACT

A tracing system includes a trace bus and compression selection circuitry. The compression selection circuitry is configured to receive a trace data input from a source of trace data and determines a compression method for a piece of trace data selected for transmission on the trace bus. The compression selection logic selects the compression method from a set of compression methods based on at least one compression criterion. The tracing system signals the compression method that was used to compress a piece of compressed trace data selected for transmission.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 09/844,672,entitled “Dynamic Selection of a Compression Algorithm for Trace Data,”filed on Apr. 30, 2001, the disclosure of which is incorporated hereinby reference in its entirety.

BACKGROUND

1. Field of the Invention

The present invention relates generally to on-chip debugging, and morespecifically to program counter (PC) and data tracing in embeddedprocessor systems.

2. Discussion of the Related Art

Computer systems process information according to a program thatincludes a sequence of instructions defined by an application program oran operating system. Typically, a program counter provides a series ofmemory addresses that are used by the processor for fetchinginstructions stored in the associated memory. In this process, theprocessor conveys the memory address to the memory over an address bus,and the memory responds over an instruction/data bus with theinstruction stored in the addressed memory location. The instructionsstored in the memory constitute the program to be executed.

Program development relies heavily on the verification of theinstructions stored in memory as well as their corresponding execution.This task represents an ever-increasing burden due to the growth insoftware complexity and the increase in processor clock speeds.Typically, these debug efforts are supported by instruction tracingtools that generate a listing of executed instructions during theexecution of a program.

Computer engineers rely on tracing results to gain a view of process anddata states while the computer system is operating. Hardware engineersrely on tracing results to determine how new computer hardwarearchitectures perform with existing operating systems and applicationprograms. Finally, software engineers rely on tracing results toidentify critical code segments and data structures.

In general, the cost of developing and debugging new software productscan be a significant factor in processor selection. A processor'sfailure to adequately support software debug efforts can result inlonger development times. This factor can easily reduce the processor'sattractiveness to particular segments of the processor industry.

The increased control and flexibility in the generation of tracing datais particularly important for the embedded processor industry. In theembedded processor industry, specialized on-chip circuitry is oftencombined with a processor core. Embedded processors typically balancenumerous design tradeoffs such as the number of I/O-port pins and thedie size costs. A reduced I/O-port pin count results in lower bandwidthfor trace data, therefore placing a premium on the efficiency of thetracing mechanism. This example is reflective of the embedded processorindustry's general need for a set of efficient, low-cost, debuggingtools.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a tracing system according to an embodiment of thepresent invention.

FIG. 2 illustrates a timing diagram of tracing signals.

FIGS. 3 and 4 illustrate tracing output from an instruction pipelineaccording to an embodiment of the present invention.

FIG. 5 illustrates a timing diagram of tracing signals from multipleinstruction pipelines.

FIG. 6 illustrates an embodiment of a first trace control register.

FIG. 7 illustrates an embodiment of a trace record including softwarestate information.

FIG. 8 illustrates an embodiment of a second trace control register.

FIG. 9 illustrates an embodiment of dynamic compression selection.

DETAILED DESCRIPTION

An embodiment of the invention is discussed in detail below. Whilespecific implementations are discussed, it should be understood thatthis is done for illustration purposes only. A person skilled in therelevant art will recognize that other components and configurations maybe used without departing from the spirit and scope of the invention.

The provision of increased control and flexibility in the tracingprocess enables hardware, software, and computer engineers toeffectively debug the operation of a computer system. These debuggingefforts are enhanced when increased visibility is provided into thehardware and software state of the processor core. This is particularlytrue when dealing with embedded processors where specialized on-chipcircuitry is often combined with the processor core. Support for thesedebug efforts is provided by an embodiment of a tracing system describedbelow with reference to FIG. 1.

In the illustrated embodiment, tracing system 100 includes on-chipcomponents identified as microprocessor core 110, trace generation logic(TGL) 120, trace control block (TCB) 130, and test access port (TAP)controller 140. TGL 120 can be embodied as part of microprocessor core110. TGL 120 is generally operative to generate program counter (PC) anddata trace information based on the execution of program code in one ormore pipelines within microprocessor core 110. TGL 120 transmits thegenerated trace information to TCB 130 via trace interface 180. Anembodiment of trace interface 180 is described in greater detail below.

In the illustrated embodiment of FIG. 1, TCB 130 is located on-chip butis external to microprocessor core 110. TCB 130 captures the traceinformation that is provided by TGL 120 on trace interface 180 andwrites the trace information to trace memory 150. The format used by TCB130 in writing trace information to trace memory 150 is implementationdependent.

It is a feature of the present invention that the specification of traceinterface 180 enables an embedded processor developer to design acustomized TCB 130. The customized TCB 130 can be designed to storetrace data in trace memory 150 in accordance with a particular set ofrequirements. For example, TCB 130 can be designed to produce trace datathat is targeted for the particular needs of trace re-generationsoftware 160.

Also included on-chip is TAP controller 140. TAP controller 140 includesinstruction, data, and control registers as well as circuitry thatenables tap controller 140 to access internal debug registers and tomonitor and control the microprocessor core's address and data buses. Inone embodiment, TAP controller 140 is based on the extended JTAG (EJTAG)specification developed by MIPS Technologies, Inc.

As noted, trace memory 150 stores trace information that is receivedfrom TCB 130. In the illustrated embodiment, trace memory 150 is locatedoff-chip. In an alternative embodiment, trace memory 150 is locatedon-chip. The decision of where to locate trace memory 150 can be basedon user requirements such as the desired size of trace memory 150. Forexample, if the desired size of trace memory 150 is too large to fiton-chip, it can be located off-chip. This flexibility in design isparticularly important in the embedded processor world where die sizecosts are significant.

The trace information stored in trace memory 150 can be retrievedthrough trace regeneration software 160. Trace regeneration software 160is a post-processing software module that enables trace reconstruction.Debugger 170 is also implemented as a software module. Debugger 170interfaces with TAP controller 140 and is generally operative to displayTAP states as well as provide high-level commands to TAP controller 140.For example, debugger 170 can be used to set breakpoints or examinecontents of certain sections of memory.

Having described the general components of tracing system 100, adetailed description of an embodiment of trace interface 180 is nowprovided. As will become apparent from the following description, traceinterface 180 enables increased flexibility in controlling the contentand accessibility of trace data within microprocessor core 110.

In describing trace interface 180, an embodiment of a set of input andoutput signals from microprocessor core 110 is described. The exampleset of output signals are described first in the context of thetransmission of trace data from TGL 120 to TCB 130. Next, the exampleset of input signals are described in the context of the transmission ofcontrol information from TCB 130 to TGL 120. It should be noted that inthis description, the “PDO_” prefix to a signal name is used to identifysignals belonging to the output interface from TGL 120, while the “PDI_”prefix to a signal name is used to identify signals belonging to theinput interface to TGL 120.

In one embodiment, the set of output signals includes the signals listedin Table 1.

TABLE 1 Output Signal Name Description PDO_IamTracing Global enablesignal for signals output from the microprocessor core PDO_InsCompInstruction completion status signal PDO_AD Trace bus for trace dataPDO_TType Specifies the transmission type for the transaction on thePDO_AD lines PDO_TEnd Indicates the last cycle of the currenttransaction PDO_TMode Indicates the transmission mode for the bitstransmitted on PDO_AD PDO_LoadOrder Indicates the out-of-order-ness ofload data PDO_Overflow Indicates an internal FIFO overflow errorPDO_PgmOrder Indicates the static program schedule of instructionsexecuting in multi-pipeline cores

The operation and content of the example set of output signals in traceinterface 180 between TGL 120 and TCB 130 is described in detail below.To illustrate the protocol by which trace information generated bymicroprocessor core 110 can be transmitted from TGL 120 to TCB 130,reference is made to the timing diagram of FIG. 2. Timing diagram 200generally illustrates the interaction of Pclk (processor clock used bymicroprocessor core 110, TGL 120 and TCB 130, PDO_InsComp[2:0],PDO_TType[2:0], PDO_TEnd, PDO_AD[15:0], PDO_TMode, and PDO_Overflow intracing out information to TCB 130.

One of the output signals that is not illustrated in timing diagram 200is PDO_IamTracing. This signal, sent out from TGL 120, indicates thatthe rest of the Out signals represent valid trace data. In effect,PDO_IamTracing represents an enable signal for the rest of the Outsignals. As will be described in greater detail below, this signal canbe used to support software control of the tracing process. Inparticular, this signal is used by TCB 130 to determine whether thetrace data output by TGL 120 is valid or not valid.

Prior to describing the interaction of output signals in timing diagram200, the usage of the PDO_InsComp[2:0] signal is first discussed. Ingeneral, PDO_InsComp[2:0] is an instruction completion status signalthat can be used as an indicator of completed instructions and theirtype in the processor's pipeline. In one embodiment, PDO_InsComp[2:0]can take on the values of Table 2.

TABLE 2 PDO_InsComp Description 000 No instruction completed this cycle(NI) 001 Instruction completed this cycle (I) 010 Instruction completedthis cycle was a load (IL) 011 Instruction completed this cycle was astore (IS) 100 Instruction completed this cycle was a PC sync (IPC) 101Instruction branched this cycle (IB) 110 Instruction branched this cyclewas a load (ILB) 111 Instruction branched this cycle was a store (ISB)

It should be noted that, in one embodiment, the PDO_InsComp signal isoutput by TGL 120 at a point in the processor pipeline at which theinstruction must complete and can no longer be killed. FIG. 3illustrates the trace tap points in relation to the illustratedembodiment of instruction pipeline 300.

As illustrated, instruction pipeline 300 includes six stages. These sixstages are labeled as fetch stage 310, decode stage 320, execute stage330, memory stage 340, align stage 350, and writeback stage 360. Asfurther illustrated, the PDO_InsComp signal is output after memory stage340, the point at which the instruction should complete and can nolonger be aborted. In general, the specific point at which aninstruction completion signal is generated is dependent upon theparticular implementation of the instruction pipeline.

Referring back to Table 2, the various values of PDO_InsComp are nowdescribed. The first PDO_InsComp value ‘000’ is associated with a NoInstruction complete (NI) indication. In one example, the NI indicationcan be used when the instruction pipeline is stalled. In anotherexample, the NI indication can be used when an instruction is killed dueto an exception.

The next set of PDO_InsComp values ‘001,’ ‘010,’ and ‘011’ areassociated with the completion of instructions within a basic block.Specifically, ‘001’ is used to signal the completion of a regularinstruction (I), ‘010’ is used to signal the completion of a loadinstruction (IL), and ‘011’ is used to signal the completion of a storeinstruction (IS). As the I, IL, or IS indication is associated with thecompletion of an instruction within a basic block, the PC value of theI, IL, or IS instruction need not be traced.

It should be noted, however, that the completion of a load instruction(IL) or a store instruction (IS) may require transmission of otherinformation to make the tracing complete. In general, the transmissionof additional information can occur in certain operating modes of thetracing process. As will be described in greater detail below, the usercan direct TGL 120, using a PDI_TraceMode signal, to output one or moreof the load/store addresses and data in addition to the PC value.

In this framework, when PDO_InsComp indicates a store in the completinginstruction, the store address and data is also transmitted providedthat the user requires those values to be traced. Similarly, whenPDO_InsComp indicates a load in the completing instruction, the loadaddress and data is also transmitted provided that the user requiresthose values to be traced. In general, if the load instruction hits inthe cache, then the trace data for the load instruction is transmittedin a similar manner to the trace data for a store instruction.

If the load misses in the cache and must go to memory, then a differenttracing process results. Regardless of whether a delay is incurred inthe completion of the load instruction, the load instruction can stillbe indicated with the appropriate PDO_InsComp value IL and the sendingof the load address. The sending of the load data, however, is deferred.

In general, tracing out load data as it becomes available avoids theneed to locally save loads that hit in the cache, while waiting forearlier loads that missed in the cache to return. FIG. 4 illustratesthis concept in relation to instruction pipeline 400. As noted above,the trace tap points for instruction pipeline 400 are located aftermemory stage 340, the point at which the instruction should complete andcan no longer be killed.

In the illustrated embodiment, the PC, load address, store address, andstore data are immediately provided to compression module 410. Loaddata, on the other hand, are provided to compression module 430 onlywhen the load data is available to load order determination module 420.

If the load data is retrieved from cache, then the load data isimmediately available to load order determination module 420. The loaddata can then be passed on to compression module 420. On the other hand,if the load data is requested from memory, then the load data is notimmediately available to load order determination module 420. As notedabove, the transmission of the load data is deferred until the load datais returned from memory.

In general, compression modules 410 and 430 are operative to compressthe trace data that is to be placed into FIFO 440, thereby awaitingoutput onto trace bus PDO_AD. As will be described in greater detailbelow, compression modules 410 and 430 can be configured to dynamicallyselect from the outputs of a plurality of compression options. Thisplurality of compression option can be selected to generate reasonablecompression ratios for a variety of expected data inputs. It should benoted that, in one embodiment, compression modules 410 and 430 can becombined into a single compression module.

Load order determination module 420 is generally operative to coordinatethe output of load data that may be delayed due to cache misses. Thedeferred transmission of load data is enabled through the output of aPDO_LoadOrder[2:0] signal that is designed to signal the out-of-ordernature of load data. In one embodiment, the PDO_LoadOrder[2:0] signalcan take on the following values of Table 3.

TABLE 3 PDO_LoadOrder Description 000 data from oldest load instruction(is in-order) 001 data from second-oldest load instruction 010 data fromthird-oldest load instruction 011 data from fourth-oldest loadinstruction 100 data from fifth-oldest load instruction 101 data fromsixth-oldest load instruction 110 data from seventh-oldest loadinstruction 111 data from eighth-oldest load instruction

As noted, the PDO_LoadOrder[2:0] signal is used to indicate theout-of-order nature of load data that is traced out. In operation, thissignal indicates the position of the load in the list of currentoutstanding loads starting at the oldest. For example, assume that theprogram issues five loads A, B, C, D, E, respectively. Table 4 belowillustrates an example of how these five loads may be traced.

TABLE 4 Load Data Data Traced Load Cycle # CacheOp Available OutPDO_LoadOrder A 1 Miss — — — B 2 Hit B B 001 (second oldest) C 3 Hit C C001 (second oldest) D 4 Miss — — — E 5 Hit E E 010 (third oldest) — k —A A 000 (oldest) — k + p — D D 000 (oldest)

For simplicity, in this example, it is assumed that the data isavailable the same clock cycle as the instruction. In reality, therewill typically be some fixed pipeline delay from instruction issue towhen the data is available from the cache after a hit.

In clock cycle 1, load A misses in the cache and goes to memory. Load Ais therefore considered outstanding. In clock cycle 2, load B hits inthe cache and is immediately available. Load B is then traced out withthe PDO_LoadOrder signal indicating that the load data is the secondoldest outstanding load. Based on the values of Table 4, thePDO_LoadOrder signal will have a value of ‘001.’ At this point, load Ais considered the oldest outstanding load.

In clock cycle 3, load C hits in the cache and is immediately available.Load C is then traced out with the PDO_LoadOrder signal indicating witha value ‘001’ that the load data is the second oldest outstanding load.At this point, load A is still considered the oldest outstanding load.Load B is not considered outstanding as it was traced out at clock cycle2.

In clock cycle 4, load D misses in the cache and goes to memory. Load Dis therefore considered outstanding. At this point, both load A and loadD are the currently outstanding loads. Load A is considered the oldestoutstanding load while load D is considered the second oldestoutstanding load.

In clock cycle 5, load E hits in the cache and is immediately available.Load E is then traced out with the PDO_LoadOrder signal indicating witha value ‘010’ that the load data is the third oldest outstanding loadbehind load A and load D.

In clock cycle k, load A returns from memory and is available. Load A isthen traced out with the PDO_LoadOrder signal indicating with a value‘000’ that the load data is the oldest outstanding load.

Finally, in clock cycle k+p, load D returns from memory and isavailable. Load D is then traced out with the PDO_LoadOrder signalindicating with a value ‘000’ that the load data is the oldestoutstanding load.

In general, the use of the PDO_LoadOrder signal enables TGL 120 to avoidhaving to include memory for storing loads that are returnedout-of-order. The loads can simply be traced out as soon as they areavailable. Out-of-order transfers of data are further described inco-pending application Ser. No. 09/751,747, entitled “ConfigurableOut-Of-Order Data Transfer in a Coprocessor Interface,” which isincorporated herein by reference in its entirety.

It should be noted that in one embodiment, if the number of outstandingloads supported by the number of bits in the PDO_LoadOrder signal isexceeded, then an overflow signal is issued. The internal buffers arethen cleared and tracing is restarted. If overflows are to be inhibited,then processor core 110 should be stalled until the outstanding loadsare satisfied before continuing.

As described, the classification of an instruction as a load or storeinstruction can influence the tracing process. Additionalcharacteristics of the instruction can also affect the tracing process.In particular, the classification of an instruction as a branchinstruction can also affect the tracing process.

As noted, the I, IL, and IS instruction classifications were associatedwith instructions that resided within a basic block of instructions.Branch instructions, on the other hand, are associated with instructionsthat were the target of a taken branch, statically predictable or not.

The completion of these types of branch instructions are signaled usingthe PDO_InsComp values (see Table 2 above) of ‘101,’ ‘110,’ and ‘111.’Specifically, ‘101’ is used to signal the completion of a regular branchinstruction (IB), ‘110’ is used to signal the completion of aload-branch instruction (ILB), and ‘111’ is used to signal thecompletion of a store-branch instruction (ISB).

The three branch-type encodings (101, 110, and 111) imply that theassociated instruction was the target of a taken branch. It should benoted, however, that the branch-type encoding can also be used when anexception is taken and when a return from exception happens since bothchange the PC value in an unpredictable way.

In general, a branch is indicated on the first instruction in a newbasic block. When this first instruction is either a load or a store,then the PDO_InsComp signal takes values ILB or ISB, respectively, toindicate the combined condition of a branch and a load or store.

As noted, some completing instructions transmit other information tomake the tracing complete. For example, if the branch was unpredictableand the unpredictability lies in the branch target address, then the PCvalue should be transmitted. If the unpredictability lies in the branchcondition (i.e., determining if the branch is taken or not), on theother hand, then the branch target PC value need not be transmitted.Here, it is sufficient to simply indicate that the branch was taken.

For ISB and ILB indications, the user may require that the targetaddress and/or data be traced along with the transmitted PC value. Inparticular, for an ISB indication, the PC value is sent first, followedby the store address, and finally the store data. For an ILB indication,the PC value and load address are sent first, followed by the load datawhen it becomes available.

In general, the tracing of the PC value is important where the PC valuecould not be statically predicted. Without this information, traceregeneration software 160 is unable to reconstruct the program executionpath.

In the present invention, the program trace reconstruction efforts areassisted through the periodic transmission of a PC value. This periodictransmission of the PC value enables trace regeneration software 160 toresynchronize itself to the program trace. Trace regeneration software160 may require resynchronization in situations where trace informationis lost (e.g., trace FIFO overflow). The transmission of generalsynchronization information beyond the PC value is described in greaterdetail below.

The periodic transmission of the PC value is signaled using aPDO_InsComp signal value of ‘100,’ which is associated with a PC sync(IPC) indication (see Table 2). The periodic output of the full PC valueenables trace regeneration software 160 to resynchronize itself with theoutput PC trace and the static program code. This full PC value isoutput approximately every synchronization period as defined by theuser. In one embodiment, TGL 120 will ensure that the synchronizationsignal does not happen on an unpredictable branch, load, or storeinstruction. The operation and control of the synchronization signalfeature is described in greater detail below.

Having defined the various values of the PDO_InsComp signal, the outputtracing process is now explained through the example of timing diagram200 of FIG. 2. Timing diagram 200 illustrates the PDO_InsComp[2:0]signal relative to Pclk. At clock cycle 1, PDO_InsComp[2:0] has a valueIB, indicating the completion of a branch instruction. In the followingdescription, the output signals are frequently described with respect totheir indication as opposed to their bit values. For example, thePDO_InsComp[2:0] signal bit value ‘101’ is also referred to by its IBvalue indication.

The value IB represents the completion of an instruction that could notbe statically predicted. Accordingly, the PC value for the branchinstruction should be traced, thereby enabling trace regenerationsoftware 160 to recreate the execution of a new block of instructions.

The PC value for the branch instruction is transmitted on the trace busPDO_AD[15:0]. In general, when a PC change, load/store address, orload/store data information needs to be traced, these pieces of traceinformation are all sent out on the same PDO_AD trace bus. In general,the width of the PDO_AD trace bus is implementation dependent. In oneembodiment, the trace bus PDO_AD is configured to be 32-bits wide.

In general, since the width of the PDO_AD trace bus may not be adequateto transmit the entire address or data in one cycle, each transactionmay take multiple cycles to transmit. A FIFO (illustrated as element 440in FIG. 4) is therefore used to hold pending transactions and values. Inone embodiment, if a transaction takes multiple cycles, then theleast-significant bits are sent first, followed by the more-significantbits. When only a byte of data is sent, it is transmitted on theleast-significant bits. In general, there is no need to indicate thevalidity since the post-analyzing software knows that the associatedinstruction is a load or store byte, and hence, will only look at thelower 8 bits of a 16-bit bus.

Distinguishing between the types of trace data that are transmitted onthe PDO_AD trace bus is accomplished using the PDO TType[2:0] signal.The PDO_TType[2:0] signal is used to indicate the type of informationbeing transmitted on the PDO_AD bus. In one embodiment, thePDO_TType[2:0] signal can take on the values of Table 5.

TABLE 5 PDO_TType Description 000 No transmission this cycle (NT) 001Begin transmitting the PC (TPC) 010 Begin transmitting the load address(TLA) 011 Begin transmitting the store address (TSA) 100 Begintransmitting the data value (TD) 101 Begin transmitting the processormode and the 8-bit ASID value (TMOAS) 110 Begin user-defined tracerecord - type 1 (TU1) 111 Begin user-defined trace record - type 2 (TU2)

As illustrated in Table 5, various data types can be output on trace busPDO_AD. A first set of trace data includes PC values (TPC), load addressvalues (TLA), store address values (TSA), and data values (TD). Thesetrace data types are identified using the PDO_Ttype signal values of‘001’ to ‘100,’ respectively.

Additional trace data beyond PC, address and data values can also betransmitted on trace bus PDO_AD. Specifically, PDO_TType signal value‘101’ is used to identify the transmission of processor mode andapplication space identity (ASID) information. As will be described ingreater detail below, the processor mode and ASID information can beincluded as part of the synchronization information that is periodicallytransmitted. This portion of the synchronization information enablestrace regeneration software 160 to identify the software state of thecomputer system being traced.

The final data types that can be transmitted on trace bus PDO_AD areuser-defined trace records TU1 and TU2. These user-defined trace recordsare identified using PDO_TType signal values ‘110’ and ‘111,’respectively. The use of user-defined trace records is described ingreater detail below.

In general, the types of trace data that are transmitted on the tracebus PDO_AD is dependent upon the type of instruction that has completed.As noted, for instructions (I) within a block, a PC value need not betransmitted. For load instructions (IL) or store instructions (IS)within a block, however, a user may decide to trace the target addressand data.

For branch instructions where there is a jump in PC, several optionsexist. In one embodiment, the following rules can be applied: (1) whenthe branch is unconditional and the branch target is predictable, IB,ILB, or ISB is used for the PDO_InsComp value, and the PC value is nottraced out; (2) when the branch is conditional, and the branch target ispredictable, IB, ILB, or ISB is used only when the branch is taken andthere is no need to trace out the PC value; and (3) when the branch isconditional or unconditional, and the branch target is unpredictable,IB, ILB, or ISB is used and the PC value is traced out using TPC for thePDO_TraceType signal.

As an example, the PC value can be transmitted (a) after a JR or JALRinstruction; (b) after a control transfer to an exception handler; (c)after a return from exception (ERET or DERET instruction); and (d) forresynchronization purposes.

Returning to timing diagram 200 of FIG. 2, the PC value is transmittedat clock cycle 1 on trace bus PDO_AD[15:0] upon the signaling of an IBsignal on PDO_InsComp[2:0]. The type of data that is transmitted ontrace bus PDO_AD[15:0] is made known to TCB 130 through the transmissionof the TPC value on PDO_TType[2:0].

The transmission of the PC value on PDO_AD[15:0] requires one clockcycle. Accordingly, the PDO_TEnd signal is asserted on clock cycle 1.Generally, the PDO_TEnd signal indicates the last cycle of the currenttransaction on trace bus PDO_AD[15:0]. This signal can be asserted inthe same cycle that a transaction is started implying that theparticular transaction only took one cycle to complete.

As illustrated in FIG. 2, timing diagram 200 also includes a PDO_TModesignal. The PDO_TMode signal indicates the transmission mode for thebits transmitted on trace bus PDO_AD[15:0]. As will be described ingreater detail below, various types of data compression can be appliedto the particular types of trace data. For example, a delta value can beused for the transmission of sequential PC values. Delta compression,however, may not be appropriate to the transmission of data values sincethose values are unlikely to be sequentially related. For those values,a form of bit-block compression may be more appropriate.

In accordance with the present invention, the PDO_TMode signal can beused to signal to TCB 130 the type of compression that has beenperformed on the trace data that is transmitted on trace busPDO_AD[15:0]. This mode information is therefore used by TCB 130 toregenerate the program flow accurately.

As noted, the types of available compression options can be dependent onthe trace data type indicated by the PDO_TType signal. Accordingly, inone embodiment, the PDO_TMode signal can have values that are dependenton the PDO_TType signal value. In an alternative embodiment, thePDO_TMode signal has values that are independent of the PDO_TType signalvalue. In other words, regardless of the type of trace data that isbeing transmitted, the PDO_TMode will have a value that identifies oneof a plurality of eligible compression options. In timing diagram 200,the PDO_TMode signal is left unspecified.

At clock cycle 2, PDO_InsComp[2:0] has a value I, indicating thecompletion of an instruction within a block of instructions. As noted,the completion of an instruction within a block does not require thetracing of the PC value. Accordingly, no transmission occurs on tracebus PDO_AD[15:0]. The no transmission state is also signaled by thePDO_TType signal with a NT value.

At clock cycle 3, PDO_InsComp[2:0] has a value IB, indicating thecompletion of another branch instruction. The PC value is thentransmitted on trace bus PDO_AD[15:0] with the data type TPC indicatedon PDO_TType[2:0]. As illustrated, the transmission of the PC valuerequires two clock cycles (3 and 4). Accordingly, the PDO_TEnd signal isnot asserted until the end of the transaction at clock cycle 4. Alsooccurring at clock cycle 4 is the signaling of value I onPDO_InsComp[2:0]. This indicates the completion of an instruction withina block of instructions and no transmission on trace bus PDO_AD[15:0] isrequired.

At clock cycle 5, PDO_InsComp[2:0] has a value IL, indicating thecompletion of a load instruction. Here, the PC value need not betransmitted. The user can specify, however, that the load address anddata be traced. With the assumption that the load hit in the cache, theload address and data is immediately available. The load address istransmitted first on PDO_AD[15:0] at clock cycles 5 and 6, and the loaddata is transmitted next on PDO_AD[15:0] at clock cycles 7-10. In bothcases, the corresponding data type is transmitted on PDO_TType[2:0]using signal values TLA and TD, respectively.

During the load address and data transmission at clock cycles 5-10,PDO_InsComp[2:0] further signals the completion of IL at clock cycle 5,I at clock cycle 6, NI at clock cycles 7-9, and I at clock cycle 10.Each of these instruction-completion indications did not require atransmission on trace bus PDO_AD[15:0]. Accordingly, the trace data FIFOdid not overflow as it waited to be cleared during the six-cycletransmission of the load address and data during clock cycles 5-10.

Timing diagram 200 illustrates an overflow condition at clock cycle 18.The overflow indication is indicated by the assertion of thePDO_Overflow signal, thereby indicating an internal FIFO overflow error.As noted earlier, FIFO 440 is used to hold values to be transmitted thatdo not fit within a single cycle of transmission.

In a FIFO overflow condition, TGL 120 indicates using the PDO_Overflowsignal that the current tracing is being abandoned due to a FIFOoverflow. In this situation, TGL 120 discards all entries in FIFO 440,and restarts transmission from the next completed instruction. It shouldbe noted that the first instruction to be signalled after the assertionof the PDO_Overflow signal should have its PC value sent as well. Ineffect, that instruction is treated as a IB, ILB, or ISB instruction.

In timing diagram 200, the internal FIFO overflow error can beattributed to the build up of trace data due to the activity in clockcycles 12-18. At clock cycle 12, PDO_InsComp[2:0] indicates completionof a branch store instruction ISB. The PC value, store address, andstore data are then transmitted on trace bus PDO_AD[15:0] at clockcycles 12-13, 14-16, and 17-18, respectively. As the trace data for theISB indication completes, however, PDO_InsComp[2:0] continues toindicate the completion of additional instructions. Specifically,PDO_InsComp[2:0] indicates the sequential completion of I, IL, IL, IS,IS, and IL instructions at clock cycles 13-18, respectively.

While the completion of instruction I at clock cycle 13 does not requiretracing of any data, the completion of the IL and IS instructions oneach of clock cycles 14-18 can require tracing of a target address anddata. Each of these pieces of trace data continues to fill FIFO 440 asthe trace data associated with the ISB instruction at clock cycle 12completes its transmission on trace bus PDO_AD. FIFO 440 thereforeeventually overflows, as shown at clock cycle 18, indicating that FIFO440 is being filled faster than it is being emptied. It should be notedthat when the PDO_Overflow signal is asserted, the values of the otheroutput signals can be disregarded. Accordingly, the PDO_TEnd signal neednot be asserted at clock cycle 18.

Upon a FIFO overflow condition, the next instruction to be signaled istreated as an IB, ILB, or ISB instruction. That is, the next instructionto be signaled is treated as if it was the start of a new block ofinstructions. In timing diagram 200, this effect is seen in the Iinstruction at clock cycle 19. If the I instruction appeared within ablock that was being traced, then the PC value does not have to betraced. In this case, however, the PC value is traced on the trace busPDO_AD[15:0] because it occurred after the assertion of the overflowsignal on PDO_Overflow.

It should be noted that it is possible for the entire program trace tobe captured under all circumstances. Trace records need not be lost.Maintaining the integrity of the entire program trace can beaccomplished using an inhibit overflow signal PDI_InhibitOverflow.

When the PDI_InhibitOverflow signal is asserted, this implies thatmicroprocessor core 110 should stall the instruction pipeline withoutoverflowing FIFO 440. Thus, if the PDI_InhibitOverflow signal isasserted, the PDO_Overflow signal will not be asserted. The instructionpipeline is restarted as soon as FIFO 440 starts emptying again.

The above description has thus far been focused on the tracing of datafor a single instruction pipeline. In accordance with the presentinvention, the tracing functionality described above can also besupported for a microprocessor core 110 that supports tracing ofinstructions executed by multiple instruction pipelines.

With multiple instruction pipelines, the tracing of instructions fromeach instruction pipeline is coordinated so that the instructions thatare issued together are also traced out together. To trace instructionsout together, TGL 120 should hold instructions that complete earlierthan other instructions that were part of the same group of instructionsthat were issued together.

It should be noted that if microprocessor core 110 implements multipleinstruction pipelines, then the “Out” signals are duplicated as manytimes as there are instruction pipelines within microprocessor core 110.For convenience, the signal names described below have a “_n” appendedto the signal name. Here, “n” is used to designate a pre-determinedpipeline number. For example, a microprocessor core with two integerpipelines may use the signals PDO_InsComp_0 and PDO_InsComp_1 torepresent the instruction completion information from each respectivepipeline.

In accordance with the present invention, TGL 120 tags instructions fromthe same issue cycle. These tagged instructions are held by TGL 120until all of the instructions in the issue group are either completed orare known to not complete (e.g., exception). The group of instructionscan then be traced out together using another “Out” signal,PDO_PgmOrder[2:0]. In one embodiment, TGL 120 includes an extra bufferwhose depth is dependent on the instruction pipeline depths.

In general, the program order signal PDO_PgmOrder[2:0] is used toindicate the static schedule of an instruction in a pipeline withrespect to instructions in other pipelines. When multiple instructionpipelines are supported, the PDO_PgmOrder[2:0] signal is provided fromTGL 120 to TCB 130. When multiple instruction pipelines are notsupported, this signal can be omitted or tied off otherwise.

The three bits of the PDO_PgmOrder[2:0] signal allows for up to eightinstruction pipelines to be used. Additional bits can also be used tosupport additional instruction pipelines.

In one embodiment, a PDO_PgmOrder[2:0] value of 0 indicates that theinstruction is the earliest in the static program image, aPDO_PgmOrder[2:0] value of I indicates that the instruction is thesecond earliest in the static program image, and so on. The lastinstruction is the one with the highest value.

To illustrate the use of the PDO_PgmOrder[2:0] signal, reference is madeto the assembly fragment in Table 6 and the timing diagram of FIG. 5.

TABLE 6 Cycle Inst. No. No. PC Instruction Pipe # 1 1 0x00400188 SWa2/$6, 0xe170($at/$1) 0 2 2 0x0040018c SW a0/$4, 0xb134(gp/$28) 0 3 30x00400190 SW a1/$5, 0xb130(gp/$28) 0 4 4 0x00400194 SW r0/$0,0x1c(sp/$29) 0 5 0x00400198 JAL 0x418d9c 1 5 6 0x0040019c OR s8/$30,r0/$0, r0/$0 0 7 0x00418d9c NOP 1 6 8 0x00418da0 JR ra/$31 1 90x00418da4 NOP 0 7 10 0x004001a0 JAL 0x411c40 1 11 0x004001a4 NOP 0

Table 6 illustrates an example of the execution of an assembly fragmentin an environment that includes a dual-issue microprocessor core. Theassembly fragment is listed in Table 6 based on its PC and Instructionvalues. For simplicity, the instructions in the assembly fragment arealso identified by an instruction number (Inst. No.). The instructionnumber values are used to identify instruction execution in timingdiagram 500 of FIG. 5.

In the dual-issue microprocessor core of the present example, the twoinstruction pipelines are referred to as instruction pipeline 0 andinstruction pipeline 1. Here, it is assumed that instruction pipeline. 0is configured to execute load/store/operate instructions, whileinstruction pipeline 1 is configured to execute branch/jump/operateinstructions. Based upon this assumption, the instructions in the codefragment of Table 6 can be assigned to a particular instruction pipelinefor execution.

As illustrated, instructions 1-4 are each store instructions. In ourexample framework, these instructions can only be executed byinstruction pipeline 0. Accordingly, instructions 1-4 are sequentiallyassigned to instruction pipeline 0 in clock cycles 1-4, respectively.Instruction pipeline 1 is not utilized until one of a brach/jump/operateinstruction is executed.

As illustrated in Table 6, instruction pipeline 1 is first utilized atclock cycle 4. At clock cycle 4, store instruction number 4 is executedby instruction pipeline 0, while jump instruction 5 is executed byinstruction pipeline 1. As further illustrated, both instructionpipelines are utilized at clock cycles 5-7. Specifically, at clock cycle5, OR instruction 6 is executed by instruction pipeline 0 and NOPinstruction 7 is executed by instruction pipeline 1; at clock cycle 6,jump instruction 8 is executed by instruction pipeline 1 and NOPinstruction 9 is executed by instruction pipeline 0; and at clock cycle7, jump instruction 10 is executed by instruction pipeline 1 and NOPinstruction 11 is executed by instruction pipeline 0.

As illustrated, both of the instruction pipelines in the microprocessorcore are used in clock cycles 4-7 to execute instructions 4-11. Toindicate the static schedule of an instruction in instruction pipeline 0relative to an instruction in instruction pipeline 1, the PDO_PgmOrder_nsignal is used. The PDO_PgmOrder_n signal is an order signal thatindicates the order of the instruction in that issue group relative tothe other instructions.

It should be noted that the addition of the PDO_PgmOrder_n signal is byitself not sufficient to enable an accurate determination of the staticprogram order. This results since each instruction can spend a differentamount of time in each instruction pipeline. In accordance with thepresent invention, TGL 120 holds all of the early completinginstructions in a group of instructions that have been issued together,and then traces the group of instructions out together in the same cyclewith the appropriate PDO_PgmOrder_n signal. This additional order signalenables trace regeneration software 160 to reconstruct the programexecution correctly.

To illustrate the usage of the PDO_PgmOrder_n signal, reference is madeto timing diagram 500 of FIG. 5. Timing diagram 500 illustrates theinstruction completion signals PDO_InsComp_0 and PDO_InsComp_1 relativeto the program order signals PDO_PgmOrder_0 and PDO_PgmOrder_1 for theassembly fragment of Table 6. Both the PDO_InsComp_0 and PDO_InsComp_1signals operate in the manner described above. Each of these instructioncompletion signals identifies the instruction that completes in aparticular cycle on that particular instruction pipeline. Forconvenience, the instruction completion signals of timing diagram 500are labeled with an instruction number in addition to the code for thetype of instruction (e.g., I, IL, IS, etc.).

At clock cycles 1-3, the PDO_InsComp_0 signal indicates the completionof instructions 1-3, respectively. As described above, each ofinstructions 1-3 represent a store instruction (IS). At clock cycle 4,both the PDO_InsComp_0 signal and the PDO_InsComp_1 signal indicate thecompletion of an instruction. Specifically, PDO_InsComp_0 signals thecompletion of instruction 4, while PDO_InsComp_1 signals the completionof instruction 5.

Significantly, the usage of the PDO_InsComp_n signals identify thecompletion of instructions 4 and 5 at clock cycle 4 does not provide anyindication of the static program order of instruction 4 relative toinstruction 5. Instead, the PDO_PgmOrder_n signals are used to identifythe static program schedule. At clock cycle 4, the PDO_PgmOrder_0 signalidentifies the first instruction in the issue group with a “0,” whilethe PDO_PgmOrder_1 signal identifies the second instruction in the issuegroup with a “1.”

As further illustrated in timing diagram 500, the relative staticprogram ordering between instructions in instruction pipeline 0 andinstruction pipeline 1 is reversed at clock cycle 6. Here, thePDO_InsComp_0 signal indicates the completion of instruction 9, whilethe PDO_InsComp_1 signal indicates the completion of instruction 8. Asinstruction 9 follows instruction 8 in the static program schedule, thePDO_PgmOrder_0 signal identifies the second instruction in the issuegroup with a “1,” while the PDO_PgmOrder_1 signal identifies the firstinstruction in the issue group with a “0.”

As thus described, the PDO_PgmOrder_n signal enables the tracing ofinstructions from multiple instruction pipelines to be coordinated.Instructions that are issued together can therefore be traced outtogether. As noted, instructions from an issue group that completeearlier are held until all of the instructions in the issue group eithercomplete or are squashed. At that point, the instructions in the issuegroup can be traced out using the PDO_PgmOrder_n signal.

Having described an example set of output signals used to transmit tracedata from TGL 120 to TCB 130, various controls of the output tracingprocess are now described. In one embodiment, the controls of the outputtracing process are effected through a set of “In” signals that aretransmitted from TCB 130 to TGL 120. As noted above, these “In” signalsare identified using the “PDI_” prefix.

In one embodiment, the set of input signals includes the example set ofsignals listed in Table 7. This set of “In” signals provides variouscontrols over the tracing process.

TABLE 7 Input Signal Name Description PDI_TraceOn This signal specifieswhether tracing is globally turned on or off PDI_TraceMode IfPDI_TraceOn is asserted or a hardware breakpoint triggers trace on, thissignal indicates what is to be traced by the core. In one embodiment,this signal can include the following values: 000: Trace PC 001: TracePC and load address 010: Trace PC and store address 011: Trace PC andboth load/store addresses 100: Trace PC and load address and data 101:Trace PC and store address and data 110: Trace PC and both load/storeaddress and data 111: Trace PC and load data PDI_G If this bit is set,implies that all processes are to be traced. If this bit is not set,then trace data is sent only for a process that matches thePDI_ASID[7:0] lines. PDI_ASID[7:0] This signal indicates to the TGLwhich application space identity (ASID) should trigger traceinformation. PDI_U This bit enables tracing in User Mode. This enablestracing if PDI_TraceOn is also asserted or the hardware breakpoint tracetriggers on, and either the PDI_G bit is set or the PDI_ASID matches thecurrent process ASID. PDI_K This bit enables tracing in Kernel Mode.This enables tracing if the PDI_TraceOn is also asserted or the hardwarebreakpoint trace triggers on, and either the PDI_G bit is set or thePDI_ASID matches the current process ASID. PDI_S This bit enablestracing in Supervisor Mode. This enables tracing if PDI_TraceOn is alsoasserted or the hardware breakpoint trace triggers on, and either thePDI_G bit is set or the PDI_ASID matches the current process ASID.PDI_EXL This bit enables tracing when the EXL (exception level) bit inthe Status register is one or when the ERL (error level) bit in theStatus register is one. This enables tracing only if PDI_TraceOn is alsoasserted or the hardware breakpoint trace triggers on, and either thePDI_G bit is set or the PDI_ASID matches the current process ASID.PDI_DM This bit enables tracing in debug mode (i.e., when the DM bit isone in the Debug register). PDI_InhibitOverflow This signal indicatethat the pipeline should be back-pressured (and stalled) instead ofallowing the trace FIFO to overflow. PDI_StallSending This signalindicates to the core that it must stop transmitting trace information.This request may be important when the TCB is in danger of over-runningits internal trace buffer. When this signal is asserted, the core mayhave to stall the pipeline. PDI_SyncOffEn This signal is an enablesignal for the PDI_SyncPeriod and PDI_OffChipTB signals. PDI_SyncPeriodThis signal is used to set the synchronization period. PDI_OffChipTBThis signal indicates that the trace data is being sent off-chip to anexternal trace memory. When this signal is not asserted, it indicates anon-chip trace buffer.

The PDI_TraceMode[2:0] signal is used to control the type of informationthat is to be traced out. In effect, the PDI_TraceMode[2:0] signal canbe used to limit the amount of data that is traced out to TCB 130. Forexample, trace data can be limited to the PC value using aPDI_TraceMode[2:0] signal value of ‘000.’ Alternatively, the data to betraced can be extended to include one or more of the load/store addressand data. The specific set of data to be traced can be specified usingPDI_TraceMode[2:0] signal values of ‘001’ to ‘111.’ In general, whilethe limitation on the types of trace data to be output reduces thevisibility into the operation of microprocessor core 110, it alsoreduces the likelihood of an overflow in trace FIFO 440 as trace data isaccumulated by TGL 120.

As noted above, a FIFO overflow can be prevented by back-pressuring andstalling the instruction pipeline. This action, however, serves toreduce the operational speed of the execution of the code bymicroprocessor core 110, thereby increasing the debugging time. Asillustrated in Table 7, the input signal PDI_InhibitOverflow can be usedto signal to TGL 120 that a FIFO overflow should be prevented.

Microprocessor core 110 may also be instructed to stall the instructionpipeline if TCB 130 is in danger of over-running its internal tracebuffer. Here, the PDI_StallSending signal indicates to TGL 120 that itshould stop transmitting trace information. When this signal isasserted, microprocessor core 110 may have to stall the instructionpipeline.

In addition to controlling the types of information that are beingtraced out, input signals can also be used to control when tracing isinitiated. In one embodiment, tracing can be initiated based upon theoperating mode of microprocessor core 110. Triggering tracing based uponthe operating mode of microprocessor core 110 can be advantageous when auser is attempting to debug an operating system interacting with a userprogram. Conventionally, this debugging process can be accomplished bytriggering tracing using breakpoints for each kernel entry. Thismethodology represents an intensive time-consuming task.

Alternatively, it is far easier to inhibit tracing when microprocessorcore 110 is operating in user mode, while triggering tracing on whenmicroprocessor core 110 enters kernel mode. The flexibility ofinitiating and/or inhibiting tracing based on the operating mode of theprocessor enables increased functionality in the design andimplementation of a debugging objective.

In the present description, the term “processor mode” generally refersto an operational characteristic of the microprocessor. In one example,a set of processor modes can be defined relative to privilege levels tosystem resources. The specific processor modes defined for amicroprocessor can be implementation dependent.

In one embodiment, the processor modes of microprocessor core 110include a kernel mode, a supervisor mode, and a user mode. Kernel moderepresents a highest system privilege. In kernel mode, registers can beaccessed and changed. The innermost core of the operating system runs inkernel mode. Supervisor mode has fewer privileges then kernel mode andis used for less critical sections of the operating system. Finally,user mode has the lowest system privilege. User mode is designed toprevent users from interfering with one another. For each of the kernel,supervisor, and user modes, access privileges to the virtual addressspace can be defined. In one embodiment, the accessibility and mappingof segments in the virtual address spaces is defined as set forth in theMIPS32™ and MIPS64™ architecture specifications.

As illustrated in Table 7, mode-based tracing for kernel mode,supervisor mode, and user mode can be enabled using input signals PDI_K,PDI_S, and PDI_U, respectively. These input signals enable a debugoperation to trigger or inhibit tracing based upon the entry or exitfrom one or more of the processor modes. For example, if the inputsignal PDI_K is asserted and PDO_U is not asserted, then tracing istriggered when microprocessor core 110 enters kernel mode from usermode. If input signal PDO_U is asserted, then tracing continues duringthe transition from user mode to kernel mode. In general, theflexibility in designing an effective debugging mechanism is enhancedthrough the selective triggering of mode-based tracing.

In addition to the mode-based controls, tracing can also be triggeredbased upon the identification of particular processes. In amulti-tasking system, each task or process has its own ASID value. TheASID value can be used in the debugging process to identify particularprocesses that need to be debugged.

As illustrated in Table 7, the input signal PDI_G is used to globallyenable tracing for all running processes. If this bit is not set, thentrace data is sent only for a process having an ASID value that matchesthe value of the PDI_ASID[7:0] signal. In other words, the PDI_ASID[7:0]signal identifies to TGL 120 which ASID should trigger the output oftrace information. As noted, this functionality is useful to assist inthe debugging of specific problematic processes.

As noted, there are several input signals such as PDI_TraceOn, PDI_G,PDI_ASID, PDI_U, PDI_K, PDI_S, PDI_EXL, and PDI_DM that enable tracingin various modes. In one embodiment, tracing is triggered on when thefollowing expression is true. In the following expression, “&&” is usedas a logical AND, “∥” is used as a logical OR, and “==” is used as anEQUALS TO condition evaluation.

(PDI_TraceOn) && (PDI_G || (PDI_ASID == current_process_ASID)) &&((PDI_U && user_mode) || (PDI_K && kernel_mode) || (PDI_S &&supervisor_mode) || (PDI_X && EXL_is_one_or_ERL_is_one) || (PDI_DM &&debug_mode))

One of the primary goals of the debugging process is to reconstruct thedynamic execution path of the program. Accurate trace reconstruction canoften be achieved through the provision to trace reconstruction software160 of the PC value at the head of each basic block. In some situations,however, trace reconstruction will be disrupted due to the absence oftrace information. For example, trace information may be lost if FIFO440 in TGL 120 overflows, thereby causing TGL 120 to empty FIFO 440before reinitiating the tracing process.

To enable trace regeneration software 160 to re-synchronize itself withthe program execution, synchronization information is output atpredictable intervals, approximately every synchronization period. Thissynchronization period is set through the use of the PDI_SyncPeriod[2:0]signal. In one embodiment, the PDI_SyncPeriod[2:0] signal takes on thevalues of Table 8.

TABLE 8 On-Chip Trace Off-Chip Trace PDI_SyncPeriod Memory Memory 000 2²2⁷  001 2³ 2⁸  010 2⁴ 2⁹  011 2⁵ 2¹⁰ 100 2⁶ 2¹¹ 101 2⁷ 2¹² 110 2⁸ 2¹³111 2⁹ 2¹⁴

As illustrated in Table 8, the synchronization period can be dependentupon the implementation of trace memory 150 (see FIG. 1). As noted,trace memory 150 can be located either on-chip or off-chip. On-chiptrace memory is typically smaller than off-chip trace memory. In oneexample, on-chip trace memory ranges from 64 bytes to 4 kbytes, whileoff-chip trace memory can be sized in excess of 64 kbytes.

Regardless of the size of trace memory 150, it is desirable to havemultiple instances of the synchronization information to be stored intrace memory 150. This ensures that in the process of writing trace datato trace memory 150, all instances of the synchronization informationwill not be eliminated when portions of trace memory 150 areoverwritten. Thus, in one embodiment, the synchronization period isselected such that, for a particular size of trace memory 150, N (e.g.,4) instances of synchronization information are expected to exist intrace memory 150.

It is a feature of the present invention that the synchronization periodcan be adjusted to accommodate various implementations of themicroprocessor. This adjustment feature is particularly important toembedded microprocessor implementations where tradeoffs in die size candictate a wide range of trace memory 150 implementations.

In one embodiment, the value of the PDI_SyncPeriod[2:0] signal is storedin a trace control register. An embodiment of the trace control registeris illustrated in FIG. 6. As illustrated, trace control register 600includes OfC field 610, SyP field 620, and Reserved field 630. OfC field610 includes a single bit that indicates whether trace memory 150 islocated on or off chip. SyP field 620 includes the three bits (see Table8) that are contained in the PDI_SyncPeriod[2:0] signal that istransmitted from TCB 130 to TGL 120.

The bits in OfC field 610 of trace control register 600 are used to setan internal counter. Tracing of the synchronization information istriggered when the internal counter overflows.

One of the primary elements of the synchronization information is thefull PC value. The full PC value is required periodically becausecompression module 410 in FIG. 4 may compress the PC values to conservetrace bandwidth. General compression of trace data is described ingreater detail below.

In one embodiment, the PC values that are output by TGL 120 arecompressed by transmitting the difference between the present PC valueand the most recently transmitted PC value. This form ofdelta-compression transmission can be reconstructed only if the previousPC value has been received. In situations where trace data is lost, fullPC values cannot be generated using the compressed PC values. Traceregeneration software 160 therefore loses its ability to reconstruct theprogram trace.

If multiple instances of the full PC value are assured to be stored intrace memory 150, then trace regeneration software 160 can reacquire astarting point for the reconstruction of the program trace. Even if thefull PC value is obtainable from trace memory 150, however, traceregeneration software 160 may still be unable to acquire an accurateview of the hardware and software state of software executing onmicroprocessor core 110.

For example, if a loop of instructions is being executed, theidentification of a full PC value does not provide an accurate pictureof the dynamic location of the PC. In other words, knowledge of the PCvalue does not enable trace regeneration software 160 to determine theiteration of the loop in which the instructions were executing.

It is a feature of the present invention that the periodically generatedsynchronization information includes information beyond a PC value. Inone embodiment, the periodically generated synchronization informationincludes one or more of the load/store addresses, ASID value, andprocessor mode information.

As illustrated in Table 7, the PDI_TraceMode signal can be used toinform TGL 120 of what types of data should be traced. If a value of‘000’ is sent, TGL 120 will output only PC trace data. In this scenario,the periodic synchronization information includes the PC and theprocessor mode/ASID value to enable trace regeneration software 160 toresynchronize itself with the program execution.

If the PDI_TraceMode signal informs TGL 120 to trace the load/storeaddresses, then the full load/store address information is traced whenthe next load/store instruction is traced. The inclusion of theload/store addresses in the periodic output has many uses that include,but are not limited to: (1) pin-pointing an iteration within a loop, (2)acquiring a full address instead of a delta (compressed) load/storeaddress, and (3) general synchronization with program execution.

More generally, it is a feature of the present invention that theperiodic transmission of synchronization information enables traceregeneration software 160 to reacquire an accurate view of the softwarestate of the computer system being traced. In that regard, the processormode and ASID value are important pieces of information that enabletrace regeneration software 160 to corroborate its assumption of thecurrent processor state.

In one embodiment, the processor mode and ASID value are traced outusing the trace record illustrated in FIG. 7. Trace record 700 includesASID field 710, processor operating mode (POM) field 720, andinstruction set architecture mode (ISAM) field 730.

ASID field 710 includes an indication of the eight-bit ASID value forthe process currently being traced. POM field 720 includes an indicationof the processor mode. In one embodiment, POM field 720 includes twobits that enables an indication of one of a kernel mode, supervisormode, user mode, and debug mode. ISAM field 730 includes an indicationof the particular instruction set architecture being used. In oneembodiment, ISAM field 720 includes two bits that enables an indicationof one of the MIPS16™, MIPS32™, and MIPS64™ architectures.

Like the PC value and the load/store address, trace record 700 istransmitted from TGL 120 to TCB 130 over trace bus PDO_AD. The tracedata type indicated using the PDO_TType[2:0] signal is TMOAS, asillustrated in Table 5 (value ‘101’).

In general, it should be noted that trace record 700 should be tracedwhenever there is a change in the ASID value. The same transaction isalso used to trace changes in the POM and ISAM.

As thus described, the input signals of Table 7 provide a mechanism foreffecting hardware control over the tracing process. It is a feature ofthe present invention that tracing can be controlled not only via theinput signals of Table 7, but also from commands from within a programitself. These commands effect control over the tracing process throughthe use of a software-settable trace control register.

In operation, the user can compile a program with the trace controlsincluded in the compiled code. The compiled code then initiates tracingon its own, without requiring the user's physical presence to initiatetracing using debugger breakpoints or other interactive trace commands.

To illustrate the advantage of this debugging feature, consider ascenario where a particular section of code of a program is beingdebugged. In accordance with the present invention, tracing can beinitiated from within the program whenever that particular section ofcode is entered. Tracing can also be stopped whenever execution leavesthat section of code. This software-trace-control feature is especiallyuseful in debugging long-running operations, such as booting anoperating system where the section of code of interest occurred hoursinto the booting process.

An embodiment of the software-settable trace control register is nowdescribed with reference to trace control register 800 of FIG. 8. Tracecontrol register 800 includes On field 801, Mode (M) field 802, Global(G) field 803, ASID field 804, ASID_M field 805, User Mode (U) field806, Kernel Mode (K) field 807, Supervisor Mode (S) field 808, EXL field809, Debug Mode (D) field 810, Inhibit Overflow (IO) field 811, Reserved(RES) field 812, User Trigger (UT) field 813, and Trace Select (TS)field 814.

In general, the software-settable fields 801-814 contained within tracecontrol register 800 enable similar trace control functionality as theinput signals illustrated in Table 7. In particular, trace controlfields 801-804 and 806-811 have direct counterparts to the equivalentsignals of Table 7. Accordingly, trace control fields 801-804 and806-811 are not described in greater detail here. The remaining tracecontrol fields, ASID_M field 805, UT field 813, and TS field 814, aredescribed below. Trace control field 812 is designated as being reserved(RES).

TS field 814 is generally operative to enable the selection betweeneither hardware or software based trace control. A TS value of oneindicates a selection of external hardware trace control using signalsgenerated by TCB 130, while a TS value of zero indicates a selection ofinternal software trace control based upon the contents of trace controlregister 800.

ASID_M field 805 is operative to indicate a mask value that is appliedto the ASID comparison. As noted, the ASID comparison is performed whenthe PDI_G signal or the bit in G field 803 is not set, therebyindicating that all processes are not being traced. The ability to maskthe ASID value provides additional flexibility in enabling the tracingof multiple processes. For example, multiple processes can be identifiedand traced based upon an identifiable set of higher-order bits in ASID_Mfield 805. In one embodiment, masked bits are identified with a “0”value, while unmasked bits are identified with a “1” value.

UT field 813 is used to indicate a type of user-trace-data (UTD) record.This UTD record is based upon the contents of a UTD register that iswritten to by software. The UTD record is created upon a write to theUTD register.

As noted in Table 5 above, the UTD record is traced out on trace busPDO_AD with PDO_TType[2:0] having a value of ‘110’ (type 1) or ‘111’(type 2). The type 1 and type 2 UTD records are reflected in the stateof the bit in UT field 813. In one embodiment, UT field can be designedto accommodate additional bits for indications of additional types ofUTD records.

It is a feature of the present invention that the UTD record enables auser to trace out any data value that can be written into the UTDregister. As such, the UTD record can include any general processorregister value, any program variable value, or any other debug-relatedinformation that is observable during program execution. Thisfunctionality enables a program to effectively trace itself withoutinteractive user commands. Flexibility in tracing intermediate values, asmall subset of data values, or specific points or aspects of programexecution is thereby improved.

As described, the principles of the present invention enable increasedcontrol and flexibility in the tracing process. These elements ofcontrol provide users with effective tools in carrying out debuggingprocedures. Greater visibility into the execution of the processingsystem is also achieved through the increased accessibility inidentifying parameters of the hardware/software state during execution.

In accordance with the present invention, performance of the tracingsystem can also be improved enough the efficient use of tracingbandwidth. Conservation of bandwidth is effected through the compressionmodules described briefly above with reference to FIG. 4. As noted,compression modules 410 and 430 are operative to compress trace datathat is to be transmitted to TCB 130.

Compression modules 410 and 430 are operative to compress a variety ofdata types. Trace information to be transmitted can include PC,load/store addresses, load/store data, processor mode information, anduser-defined data values. These different types of trace data areidentified during transmission on trace bus PDO_AD using thePDO_TType[2:0] signal defined in Table 5.

As noted, in one embodiment, the compression option that is applied tothe trace data can be chosen from a set of compression options that aredefined for the particular type of trace data that is being traced. Inother words, the PDO_TMode signal that defines the type of compression(or mode of transmission) being used is dependent upon the value of thePDO_Ttype signal. In one embodiment, the PDO_TMode signal takes on thevalues of Table 9 relative to the PDO_TType signal values.

TABLE 9 PDO_TType PDO_TMode 000 Reserved 101 001 0: delta from last PCvalue 1: compression option A (full address) 010 0: delta from last dataaddress of that type 011 1: compression option B (full address) 100 0:compression option C (full data) 110 1: compression option D (full data)111

In the illustrated embodiment of Table 5, a PDO_TType value of ‘001’ isused when transmitting the PC value. For this type of trace data, twocompression modes are defined. In the first compression mode identifiedby a PDO_TMode value of ‘0,’ the delta value relative to the PC value ofthe previous instruction is used. This form of compression is efficientand desirable when the PC value is sequentially related to the previousPC value. If the PC value is independent of the previous PC value, thenan alternate compression option A can be identified by a PDO_TMode valueof ‘1.’ Compression option A can represent any compression option thatis expected to produce significant compression results when applied to afull PC address value.

As further illustrated in Table 9, delta compression can also be usedfor PDO_TTypes with a value of ‘010’ (load address) and ‘011’ (storeaddress). These trace data types can also appear in a sequential patternor exhibit a fair degree of locality and can therefore take advantage ofdelta compression. Alternately, the PDO_TMode signal can identify analternate compression option B to be used for load/store addresscompression.

Finally, PDO_TTypes ‘100’ (data value), ‘110’ (user-defined tracerecord—type 1) and ‘111’ (user-defined trace record—type 2) define fulldata values. Compression of these full data values is accomplishedthrough the PDO_TMode signaling of either compression option C orcompression option D. In general, the choice of compression options Cand D can be based upon some expectation of the type of data that is tobe compressed.

In the embodiment described above, the selection of compression optionsA-D can be based upon some expectation of the characteristics of thedata that is to be compressed. For example, delta compression wasselected as one of the possible compression options for data that wasreasonably believed to have some sequential relation to priortransmissions.

In an alternative embodiment, a set of compression options can beapplied to the trace data regardless of the characteristics of theexpected trace data. In this embodiment, a set of compression optionscan be selected to cover a wide range of potential compressionscenarios. Each of the selected compression options can then be appliedto a particular piece of trace data without prior knowledge of theexpected compression quality of the result. For that particular piece oftrace data, the best compression result from the set of compressionoptions can be selected. The selected compression option can then besignaled to TCB 130 using the PDO_TMode signal. In general, thiscompression mechanism effects a dynamic selection of the compressionoption for the trace data.

An embodiment of the logic that embodies a dynamic compression selectionoption is illustrated in FIG. 9. It should be noted that compressionselection logic 900 can be incorporated into one or more of compressionmodules 410, 430 of TGL 120.

Compression selection logic 900 includes compression modules 910 ₁-910_(N). Each of compression modules 910 ₁-910 _(N) operates on the samepiece of trace data that is to be transmitted. In general, compressionmodules 910 ₁-910 _(N) can represent any set of compression methods thatcan be used to compress a set of bits. In one embodiment, compressionmodules 910 ₁-910 _(N) can be based on such compression-based methods asdelta compression, bit-block compression, run length encoding, variablebit-block compression, or the like. Bit-block compression, run lengthencoding, and variable bit-block compression are described in greaterdetail in Faloutous et al., “Description and Performance Analysis ofSignature File Methods for Office Filing,” ACM transactions on OfficeInformation Systems, Vol. 5, No. 3, July 1987, pages 237-257, which isherein incorporated by reference in its entirety. Other bit compressionmethods can also be used by the present invention.

As noted, each of compression modules 910 ₁-910 _(N) is operative togenerate a compression result for the input trace data. The trace datamay represent a PC value, a load/store address value, a load/store datavalue, processor mode information, or a user-defined data value.Regardless of the trace data type, each of compression modules 910 ₁-910_(N) will produce a result that is forwarded to compression selectormodule 920.

Compression selection module 920 is operative to compare the outputs ofcompression modules 910 ₁-910 _(N) to determine which compression optionmost efficiently compresses the input trace data. In this process, thedynamic selection of the compression outputs ensures that an efficientcompression mechanism has been selected for each piece of trace data.

The compression method selected is then signaled to TCB 130. As noted,this signaling can be based on a PDO_TMode signal. A single PDO_TModesignal can be used to indicate the selection of one of the compressionmethods embodied in compression modules 910 ₁-910 _(N). It should benoted that in one embodiment, one of the compression methods is selectedas the default form of compression.

In addition to implementations of the invention using hardware, theinvention can be embodied in a computer usable medium configured tostore a computer readable program code. The program code causes theenablement of the functions or fabrication, or both, of the inventiondisclosed herein.

For example, this can be accomplished through the use of generalprogramming languages (e.g., C, C++, etc.), hardware descriptionlanguages (HDL) including Verilog HDL, VHDL, Altera Hardware DescriptionLanguage (AHDL) and so on, or other programming and/or circuit (i.e.,schematic) capture tools available in the art.

The program code can be disposed in any known computer usable (e.g.,readable) medium including semiconductor memory, magnetic disk, opticaldisc (e.g., CD-ROM, DVD-ROM, etc.) and as a computer data signalembodied in a computer usable (e.g., readable) transmission medium(e.g., carrier wave or any other medium including digital, optical oranalog-based medium). As such, the code can be transmitted overcommunication networks including the Internet and intranets.

It is understood that the functions accomplished by the invention asdescribed above can be represented in a core which is embodied inprogramming code and transformed to hardware as part of the productionof integrated circuits. Also, the invention may be embodied as acombination of hardware and software.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof. Thus, it is intended thatthe present invention cover the modifications and variations of thisinvention provided they come within the scope of the appended claims andtheir equivalents.

1. A tracing system, comprising: a trace bus; and compression selectioncircuitry, said compression selection circuitry configured to receive atrace data input from a source of trace data and determine a compressionmethod for a piece of trace data selected for transmission on said tracebus, said compression selection circuitry selecting the compressionmethod from a set of compression methods based on at least onecompression criterion; said tracing system signaling the compressionmethod that was used to compress a piece of compressed trace dataselected for transmission; wherein said tracing system signals the tracedata type of said selected piece of compressed trace data.
 2. Thetracing system of claim 1, wherein said compression selection circuitryimplements a set of different compression methods in parallel on saidpiece of trace data to generate a corresponding set of pieces ofcompressed trace data compressed by different compression methods, saidcompression selection circuitry selecting a piece of compressed tracedata for transmission based on comparing compression results of said setof pieces of compressed trace data compressed by different compressionmethods.
 3. The tracing system of claim 2, wherein said tracing systemselects the piece of compressed trace data compressed by the compressionmethod that has the best compression efficiency.
 4. The tracing systemof claim 2, wherein said tracing system selects the piece of compressedtrace data compressed by the compression method based on compressionquality.
 5. The tracing system of claim 2, wherein said tracing systemselects the piece of compressed trace data compressed by the compressionmethod based on a level of trace data compression.
 6. The tracing systemof claim 1, wherein said source of trace data has a plurality ofdifferent trace data types.
 7. The tracing system of claim 1, whereinsaid compression selection circuitry comprises: a plurality ofcompression modules that each receive input trace data from a source oftrace data having a plurality of different trace data types, each ofsaid plurality of compression modules implementing a differentcompression method to compress a piece of trace data such that saidplurality of compression modules produces a corresponding plurality ofpieces of compressed trace data for each piece of input trace data; anda compression selector module coupled to said plurality of compressionmodules to compare compression results of said plurality of pieces ofcompressed trace data and select the piece of compressed trace data thatexhibits a desired level of trace data compression for transmission onsaid trace bus.
 8. The tracing system of claim 1, further comprising aFirst In First Out (FIFO) buffer that is coupled to said compressionselection circuitry, said FIFO buffer storing said selected piece ofcompressed trace data prior to transmission on said trace bus.
 9. Thetracing system of claim 1, wherein said set of different compressionmethods includes one or more of delta compression, hit-blockcompression, run-length encoding, and variable bit-block compression.10. A computer readable storage medium, comprising: computer-readableprogram code stored on said computer readable medium implemented as asemiconductor memory, magnetic disk or optical disk, saidcomputer-readable program code causing a computer to describecompression selection circuitry for a tracing system, said compressionselection configured to receive a trace data input from a trace bus anddetermine a compression method for a piece of trace data selected fortransmission on said trace bus, said compression selection circuitryselecting the compression method from a set of compression methods basedon at least one compression criterion; wherein said tracing systemsignals the trace data type of said selected piece of compressed tracedata.
 11. A tracing method, comprising: compressing a piece of tracedata having one of a plurality of different trace data types using aplurality of compression circuitry modules to produce a correspondingplurality of pieces of compressed trace data; selecting one of saidplurality of pieces of compressed trace data based on at least onecompression criterion; outputting said selected piece of compressedtrace data on a trace bus; and signaling the compression method used toproduce said selected piece of compressed trace data; comprisingsignaling the trace data type of said selected piece of compressed tracedata.
 12. The tracing method of claim 11, wherein said selectingcomprises comparing compression results of said plurality of pieces ofcompressed trace data and making a selection based on compressionefficiency.
 13. The tracing method of claim 11, wherein said selectingcomprises determining which of said plurality of pieces of compressedtrace data exhibits a desired level of trace data compression andchoosing one of said plurality of pieces of compressed trace data basedupon said determination.
 14. The tracing method of claim 11, whereinsaid compressing includes compressing said piece of trace data with oneor more of delta compression, bit-block compression, run lengthencoding, and variable bit-block compression.
 15. The tracing method ofclaim 11, wherein said selecting is based on a default selection. 16.The tracing system of claim 11, wherein said set of differentcompression methods includes one or more of delta compression, bit-blockcompression, run-length encoding, and variable bit-block compression.